The problem of determining the position of the normal shock in the inlet of a supersonic jet engine is one which has received considerable attention. If the shock is uncontrolled, the engine can "unstart." The result is a loss of engine power. Several technologies have been used to sense the position of the shock. The oldest is a set of pressure taps on the side of the inlet which measure the change in pressure due to the shock and from the position of the pressure shift measure the shock position. This information is then used in a feedback loop to control the inlet configuration. The pressure signature is obscured both by the shock boundary layer interaction and by the fact that the shock is not planar. Other methods have been tried experimentally, including the construction of a shadowgraph to detect the shock location.
Lidar/Laser anemometry is a well-established method of sensing air flow velocities which can operate at a point or along a line it has the advantage that there is no physical probe at the point where the velocity is being made.
In the patent literature, U.S. Pat. No. 5,164,784 assigned to The Boeing Company is illustrative of a Lidar system and method used to measure relative air velocity as a function of a Doppler shift in the frequency of light reflected from aerosol particles in the fluid.
U.S. Pat. No. 5,072,612 assigned to The Boeing Company is illustrative of a system for determining the position of a normal shock in a supersonic inlet of an aircraft power plant by projecting a sheet of light across the inlet and observing the bending of a portion of the sheet by the shock.
In the literature, a conference paper titled "Laser Velocimetry Applied to Transonic and Supersonic Aerodynamics," by D.A. Johnson, et al., appears in AGARD Conference Proceedings No. 193 on "Applications of Non-Intrusive Instrumentation in Fluid Flow Research," 3-5, May 1976, at pages 3- to 3-2. The D.A. Johnson, et al., paper shows two laser beams projected into the flow and the difference in Doppler shift is sensed to compute velocity. The region of space over which flow velocity is measured is small, being nearly a point measurement. Velocity field, including the shock wave, is measured at a point in time, in contrast to the invention hereinafter described spanning the shock.